1. Field of the Invention
The present invention relates generally to a receiver apparatus in a multi-user communication system and a control method thereof, and more particularly, to a receiver apparatus that can minimize a processing time delay and can reduce the complexity through interference cancellation processing for multiple user signals and iterative decoding processing.
2. Description of the Related Art
Portable phone service was introduced in the 1980's, and experienced explosive expansion in the early 1990's. In order to efficiently use limited frequency resources, analog-type portable phone service was replaced with digital-type portable phone service. Third-generation mobile phone service that enables the user to perform various data communication with anybody regardless of time and place is expected to be provided in the near future.
The digital portable phone systems include the Global System for Mobile communication (GSM) using the Time Division Multiple Access (TDMA) scheme, which is commercialized in Europe, and IS-95 using the Code Division Multiple Access (CDMA) scheme, which is adopted and provided as a standard in North America and Korea. The IS-95 provided in Korea is characterized by a large accommodation capacity and a soft handover between base stations, which are the advantages of the CDMA scheme, and is highlighted as a basic form for the standard of the third-generation portable phone method expected to be provided in the future.
Since commercial use of the digital portable phone service began, various studies have been made to expand the receptive capacity of the system, which is the most basic requirement. Riding on such a technical requirement, various methods for increasing the receptive capacity of a CDMA system have been introduced in the last several years. Studies about an interference cancellation method, which is one of the introduced methods, have been made and are being made from various viewpoints.
While early studies focused on an analysis for an optimum multi-user detector, more recent studies have given greater attention to how to actually implement an interference cancellation method. With the development of hardware technology, some interference cancellation methods are expected to be applied to the third-generation mobile communication system.
Generally, interference cancellation methods are classified into a Serial Interference Cancellation (SIC) scheme, a Parallel Interference Cancellation (PIC) scheme, and a Hybrid Interference Cancellation (HIC) scheme. The SIC scheme arranges the interference signals of multiple users in order of power, cancels the interference signals step by step. The SIC scheme can provide a superior detection result for a user signal having high power than other schemes, but has a disadvantage in that a delay time for interference cancellation becomes longer because it is necessary to cancel the respective user signals one by one.
The PIC scheme cancels interference signals in parallel with respect to all user signals, and thus has an advantage in that a delay time is significantly reduced as compared with the SIC scheme. However, according to the PIC scheme, since many interference signals are estimated and canceled at one time, a large error occurs when there is an inaccurately estimated interference signal, thereby degrading the performance. Finally, the HIC scheme combines the SIC scheme and the PIC scheme in order to appropriately utilize the advantages of both schemes.
When an interference cancellation scheme is applied to a Wideband CDMA (WCDMA) system taking a channel code into consideration, it is preferable to use a powerful channel code because the gain of a channel code is directly related to the gain of the interference cancellation scheme.
As full-scale multimedia services, such as images and a wireless Internet, are provided as mobile communication services, it becomes necessary to achieve low Bit Error Rate (BER) performance as well as high-speed transmission. To this end, research about error correction schemes and performance enhancement has been actively conducted, so that a turbo code has been adopted as an error correction code for the next-generation mobile communication systems, such as High Speed Downlink Packet Access (HSDPA), Wireless Broadband (WiBro), etc. The turbo code basically has a structure obtained by concatenating convolutional codes in a parallel fashion. This is to apply two or more sequences, such as component codes, having mutually different arrangements.
The originally-studied parallel concatenation code, which is used in such a dual coding scheme as to code a sequence to be transmitted by applying a code, and to again code the coded sequence by applying another code, differs from a newly proposed turbo code. The newly proposed turbo code creates a second sequence by changing only the arrangement of a first sequence and to apply the first and second sequences to mutually different coders.
The turbo code uses a soft-output iterative decoding scheme as a decoding scheme. Since the goal of the turbo code decoding is to improve the performance by exchanging information about each bit within a decoding period and using the information at the next decoding in order to improve the performance, it is necessary to obtain a soft output in a turbo code decoding step. To this end, a Maximum A Posteriori (MAP) algorithm is used.
The performance of a turbo code is determined by the number of memories in a decoder, a block size, a type of interleaving, a decoding algorithm of the decoder, the number of iterations, the number of processing bits used for an internal metric, etc. Error correction codes are roughly classified into two types, that is, into a Low Density Parity Check (LDPC) code series and a turbo-likely code series. The LDPC series has the form of a parity check matrix, and the turbo-likely code series has a form concatenated through an interleaver.
In codes using the iterative decoding, a predetermined maximum number of decoding iterations is important, and functions to determine the decoding throughput of the entire system. Even in the case of a decoder employing an iterative decoding stop criterion algorithm, a maximum number of decoding iterations predetermined in the system is still important because it is possible to iterate the decoding as many as the maximum number of decoding iterations according to frame data to be decoded.
FIG. 1 is a block diagram illustrating the configuration of a conventional turbo decoder, which basically includes a first internal decoder 101, an interleaver 102, a second internal decoder 103, a de-interleaver 104, and a hard decision unit 105.
Explaining the operation of the conventional turbo decoder in short, values input to the first and second decoders 101 and 103 include a system input signal, first and second additional input signals, and an external information value. When the system input signal, the first additional input signal, and the external information value are input to the first decoder 101, the input signals are subjected to a first decoding process through the first internal decoder 101, so that a soft decision value is output. The soft decision value obtained through the first decoding process passes through the interleaver 102, and is then input to the second decoder 103 together with the second additional input signal, thereby being again subjected to a decoding process.
Such a decoding process is iteratively performed until a desired performance has been obtained. Thus, there are essential problems in that, as input signals are subjected to iterative decoding processes and pass through the interleaver 102 and de-interleaver 104, the decoding delay time is longer and the complexity increases. Various apparatuses and methods for reducing the decoding delay time of the problems were proposed, especially, apparatuses and methods for establishing the number of iterations and terminating a decoding procedure based on the number of iterations.